Size-dependent hydrogen bonds of cluster ions between phenol cation radicals and water molecules: A molecular orbital study

The molecular structures and vibrational frequencies of [C6H5OH-(H2O)(n)](.+)(n = 1-4) are studied by employing ab initio molecular orbital methods. Since the hydrogen bond between phenol cation radicals and water molecules is much stronger than that of the neutral phenol-water system, the position of the proton of phenol cation radicals depends on the number of water molecules in the clusters [C6H5OH-(H2O)(n)](.+). Although the stable structure of [C6H5OH-(H2O)(n)](.+) varies depending on the method used in the calculation, the result obtained with the B3LYP density functional method gives good agreement with the experimental IR spectra. The proton-nontransferred form is found to be mast stable for n = 1 and n = 2 clusters. In the cases of n greater than or equal to 3, the most stable structures are the proton-transferred form. There are two types of structures obtained for n = 3 clusters, where the branched form is more stable than the chained form. The optimized structures for n = 3 and n = 4 clusters show that the H3O+ moiety prefers to interact with the phenoxy radical and two water molecules.